1. Field of the Invention
The present invention relates to a digital image display apparatus for displaying an image by converting a pixel value of a digital image data into brightness by means of a display window, which can suitably set a window level and a window width as parameters concerning pixel value-to-brightness conversion, i.e., the display window, in accordance with characteristic features of the digital image data.
2. Description of the Related Art
An example of the image display apparatus of this type is an apparatus for displaying a medical image such as a CT image or an MR image. When the medical image data is to be displayed so as to be observed or photographed on a film as a picture, each pixel value of the image data must be converted into brightness by a predetermined nonlinear conversion function. The conversion function is nonlinear because the dynamic range of the medical image is wider than that of a display screen. If all of the pixel values included in the pixel value range of the digital image data are converted into brightness by using a linear conversion function, no clear image can be displayed. This conversion function is called the display window having the window level and the window width as parameters.
In a conventional system, an operator manually adjusts the window level and the window width while monitoring the image displayed on the screen and sets the display window (conversion function) which allows a necessary portion in the image to be displayed with optimum contrast and brightness This operation becomes cumbersome for the operator if the operator must adjust the window level and width for a number of images
In an MRI apparatus or a CT apparatus, therefore, it is attempted to preset standard values of the window level and the window width for each scanning condition and read out the corresponding preset standard values in accordance with the scanning condition of each scan, thereby automatically setting the window level and the window width.
Further, it is proposed to set the display window in accordance with the characteristic features of the image data. An example of such an apparatus is an image display apparatus disclosed in Japanese Patent Disclosure No. 63-84526. This apparatus forms a histogram as shown in FIG. 1A, concerning a pixel value from image data and calculates a threshold level Th from the maximum frequency H.sub.max, e.g., 0.05 .times. H.sub.max, thereby determining a range (Wl to Wh) having a frequency with a level Th or more. The apparatus determines the window width WW (Wl to Wh) and the window level WL (=(Wh+Wl)/2) for display of the image, from the lower pixel value limit Wl and the upper pixel value limit Wh as shown in FIG. 1B, thereby forming the conversion function or the display window for pixel value-to-brightness conversion.
This conventional arrangement, however, has the following problem. That is, since the display window is set on the basis of only the threshold level obtained from the maximum frequency in the histogram, it often does not coincide with a display window set by an operator. If the automatically set window is not suitable for observation, the operator must adjust the automatically set window, resulting in a cumbersome operation.